1. Field of the Invention
The present invention relates to a method and apparatus for generating optical pulse signals used in optical communications. The invention particularly relates to a method and apparatus for generating optical pulse signals that uses an active mode-locking laser to generate uniform pulses.
2. Description of the Prior Art
One technique of increasing the amount of information that can be transmitted by an optical communications system involves the use of millimeter-wave modulated lightwave. This requires a method of stably generating millimeter-wave modulated lightwave. One such method that has drawn attention uses a mode-locked laser, more specifically, a method of modulating a laser beam in a laser resonator. This method uses a phase modulator (PM) provided in the laser resonator, an isolator (I) and a Fabry-Perot etalon (FP), forming a configuration that is herein referred to as a xe2x80x9cPM-I-FPxe2x80x9d system. If the free spectral range (FSR) of the Fabry-Perot etalon is set at Kfm, where fm is modulation frequency and K is the order of the side band of the modulation, the result is a promising method that can generate optical pulses of repetition frequency Kfm.
With this method, the phase modulation index can be increased to generate pulses of repetition frequency 2 Kfm, but the pulse waveform loses its uniformity, as has been reported by Abedin et al. (Abedin, Onodera, Hyodo, xe2x80x9cHigher Order FM Mode Locking for Pulse-Repetition-Rate Enhancement in Active Mode-Locking Lasers: Theory and Experiment,xe2x80x9d IEEE Journal of Quantum Electronics, Vol. 35, No. 6, June 1999).
FIG. 4 shows a first example of a prior-art configuration described by Abedin et al that uses a PM-I-FP system arrangement. With this configuration, when phase-modulated signal frequency fm, order K and the Fabry-Perot etalon FSR have the relationship shown by the following equation 1, the optical pulses circulate in the direction PM-I-FPxe2x86x92optical amplifierxe2x86x92couplerxe2x86x92PM-I-FP, and optical pulse train outputs of repetition frequency fp=Kfm are extracted through the coupler.
FSR=Kfmxe2x80x83xe2x80x83(1)
Here, the PM-I-FP system transfer function and transmission are functions of time t; if these are denoted as M(t) and p(t) respectively, then p(t)=|M(t)|2. Also, when xcfx89pt=2 xcfx80Kfm, it is known that transmission p(t) assumes the relative maximum value when it is an even multiple of xcfx80, xcfx89pt=0, 2xcfx80, . . . , (0 (zero) phase state) and when it is an odd multiple of xcfx80, xcfx89pt=0, 3xcfx80, . . . , (xcfx80 phase state). FIG. 5 shows the transmission of a K=2 PM-I-FP system with respect to various phase modulation indices xcex94. Here, xcfx89m=2 xcfx80fm, where xcfx89m is the angular frequency of the phase-modulated signal. Generally, the transfer function of a PM-I-FP system takes a different value in the 0 phase state and the xcfx80 phase state. Therefore, when the pulses pass the PM-I-FP system in either the 0 phase state or the n phase state. Uniform optical pulses are being generated in a mode-locked state. There are cases where, depending on the combination of the order K and the phase modulation index xcex94, the pulses are transmitted by the PM-I-FP system in both phase states, but in most such cases the oscillations are unstable and the pulse waveforms are not uniform The reason for this is that the transfer function M(t) in the 0 phase state is not the same as the transfer function M(t) in the xcfx80 phase state.
Shikatani et al. proposed a method of generating pulses using two PM-I-FP systems connected in series, with each Fabry-Perot etalon having the same free spectral range Kfm and each phase modulator having the same modulation frequency fm. In addition, the modulation signal phase was adjusted to an appropriate setting. This made it possible to generate pulses at the repetition frequency 2 Kfm that were shorter than those generated with a configuration that used just one PM-I-FP system (Shikatani et al., xe2x80x9cMethod of generating double-pulses using active mode-locking laser with compensation filter,xe2x80x9d Proceedings of the 2000 Electronics Society Conference of The Institute of Electronic, Information and Communication Engineers, C-4-29, page 377).
FIG. 6 shows a second example of a prior-art configuration according to Shikatani et al., which uses two PM-I-FP systems. Compared to FIG. 4, the configuration of FIG. 6 has another PM-I-FP system disposed within a ring laser. The optical pulses circulate in the direction first PM-I-FP systemxe2x86x92second PM-I-FP systemxe2x86x92optical amplifierxe2x86x92couplerxe2x86x92first PM-I-FP system, and optical pulse trains are extracted via the coupler. The relationship between the FSR, fm and K is that of equation 1. When At is the time taken for optical pulses to propagate from the first PM-I-FP system to the second PM-I-FP system and the phase difference of the phase modulation signal applied to the first and second PM-I-FP systems is set as shown in equation 2, the repetition frequency of the output pulses is 2 Kfm.                     φ        =                              π            K                    -                      2            ⁢            π            ⁢                          xe2x80x83                        ⁢                          f              m                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢            t                                              (        2        )            
FIGS. 7, 8 and 9 show examples of the modulation signals and transmission values in the case of FIG. 6. In the case of FIG. 7, the solid line indicates the modulation signal of the first PM-I-FP system and the broken line indicates the modulation signal of the second PM-I-FP system.
FIG. 8 shows transmission plotted against time, with the transmission of the first PM-I-FP system being denoted by the solid line and the transmission of the second PM-I-FP system being denoted by the broken line. FIG. 9 shows the composite transmission of the first and second PM-I-FP systems, plotted against time. When pulses of repetition frequency 2 Kfm pass through the first PM-I-FP system, the system is in the 0 phase state (or xcfx80 phase state), and when the pulses pass through the second PM-I-FP system, the system is in the phase state (or 0 phase state). Therefore, all pulses receive the same amount of modulation while circulating through the optical path in the resonator one time. This is also shown by the transmission curve in FIG. 9. The repetition frequency of the transmission is 2 Kfm. The system is characterized not only by the fact that repetition frequency of the output pulses is 2 Kfm, but also by the narrow pulse width and the absence of chirp modulation.
However, in the case of the above method proposed by Shikatani et al., the FSR of the Fabry-Perot etalon of each of the two PM-I-FP systems has to be the same. However, since the structural materials of the Fabry-Perot etalons are sensitive to the environmental temperature change and mechanical distortion, it is not easy to maintain plural Fabry-Perot etalons at the same FSR. In order to maintain the same FSR, it is necessary to use thermostat or the like to rigorously control the temperature and to situate the system in a location that is free of vibration.
An object of the present invention is to provide the functions of a configuration that has two PM-I-FP systems, each with its own Fabry-Perot etalon, by using a configuration having just one Fabry-Perot etalon which is used by both PM-I-FP systems, thus eliminating the need to maintain plural Fabry-Perot etalons at the same FSR, which is a problem of the prior art configurations.
In accordance with the present invention, the above object is attained by a method of generating optical pulses using an active mode-locking laser that includes first modulation means, optical selection means, second modulation means and amplification means, the method comprising: a step of using the first modulation means to modulate a lightwave; a step of selecting a modulation lightwave signal having a high frequency component by passing a lightwave through the optical selection means in a first direction; a step of using the second modulation means to modulate a lightwave; a step of selecting a modulation lightwave signal having a high frequency component by passing a lightwave through the optical selection means in a second direction; a step of amplifying the lightwave selected by the above step; and a step of supplying the amplified lightwave to the first modulation means.
With respect to the first and second modulation means, modulation has to be effected using a signal having the same frequency, and there also has to be a phase differential between the modulation signals arising in the propagation of the lightwave between the modulators. Therefore, the above method also comprises applying signals to the first and second modulation means that have the same frequency and a predetermined phase differential.
Plural optical paths can be arranged in a single Fabry-Perot etalon, but the most stable operation is provided by a configuration in which the same optical path is used bi-directionally. Therefore, the above method also comprises the first and second directions of the optical selection means being mutually reversed directions, and a region used in common by an optical path disposed in a first direction and an optical path disposed in a second direction.
The above object is also attained by an optical pulse generation apparatus comprising: an optical amplifier; an optical wave-guide; first modulation means; a configuration for selecting a modulation lightwave signal having a high frequency component by passing a lightwave in a first direction through an optical selection means for selecting a modulation lightwave that includes a high frequency component; second modulation means; a configuration for selecting a modulation lightwave signal having a high frequency component by passing a lightwave in a second direction through the optical selection means for selecting a modulation lightwave that includes a high frequency component; and a configuration for amplifying a lightwave thus selected and supplying the amplified lightwave to the first modulation means.
With respect to the first and second modulation means, modulation has to be effected using a signal having the same frequency, and there also has to be a phase differential between the modulation signals arising in the propagation of the lightwave between the modulators. Therefore, the above apparatus also comprises a configuration whereby signals are applied to the first and second modulation means that have the same frequency and a predetermined phase differential.
Plural optical paths can be arranged in a single Fabry-Perot etalon, but the most stable operation is provided by a configuration in which the same optical path is used bi-directionally. Therefore, the above apparatus also comprises the first and second directions of the optical selection means being mutually reversed directions, and a region used in common by an optical path disposed in a first direction and an optical path disposed in a second direction.